1. Field of the Invention:
The present invention relates to a method and apparatus for generating a spatially stable, small diameter light beam over a long distance. This small diamter light beam defines a straight reference line which is useful for alignment and measurement purposes.
2. Prior Art
In many instances there arises the need to produce a straight reference line against which objects are aligned or straightness of motion is measured. A common tool is the "straight edge" comprised of a rigid material fabricated with one flat side. There is, however, a practical limit on how long such a "straight edge" can be, which is about one meter because of warping due to thermal gradients and gravity sag.
An alignment telescope can also serve as a reference line. These can be used over long distances but they require mechanical refocusing for different distances which may change the direction of the straight reference line. The stability of the reference line also depends on how well the alignment telescope is mounted.
A laser beam can also be used to generate a straight reference line. However, because of the inherent divergence of a laser beam, the diameter of the reference line cannot be kept sufficiently small over long distances. It is desirable to have a small diameter since the tolerance to which its center can be found is proportional to its diameter. Therefore, for high precision metrology, a small diameter is preferred.
The present invention provides a small diameter light beam over long distances by using the unique beam produced when an incident wave is diffracted by a sphere. In one embodiment of the instanct invention, the straight reference line is defined by the direction of propagation of a light wave and the center of the diffracting sphere. In another embodiment of the instant invention, the light beam is spatially stabilized by fixing it at two points, since two points define a straight line. One point is the center of the diffracting sphere and the other point is the center of a detector, e.g. a four quadrant segmented photodetector. By properly processing the output of of the detector, the incident wave can be steered to keep the beam centered on the detector.